The wood is mainly going to keep the pressure on the stringer from breaking through the glass. You don’t need to spackle before you glue the wood on. Use Gorilla glue or another foaming PU glue, but don’t use a wood glue, you can use an epoxy. Also don’t let the glue spill out from the edge of the wood. That will create a hard spot and sanding it will make an uneven area. You also don’t need to lay fiberglass under the wood. Once the edge of the wood is sanded smooth to the foam, you can glass with 2 layers of glass. I would do 2 layers on the bottom too, the foam is too soft to make a strong shell with one layer of glass. I also put wood in the foam where the fin boxes will be. I put 1/2" pieces running perpendicular to the fin box about 4 inches long. Sometimes 2 pieces to extra strength. This will keep the boxes solid. I have had boxes tear out from just making a hard turn without the extra support. Or, you can use boxes that are set before glassing like Futures or FCS fusion and FCS 2. I like boxes because I’ve had boards delaminate under the fin and then it’s a nightmare repairing that.
I just use small bricks like little paver bricks to hold down the wood for a single width of wood. When I vac bag wood onto foam, I put a layer of glass under the wood, but I create a skin to cover the whole top. I use thin masking tape to hold the wood pieces together. I tape all the edges tightly together and put the tape side against the board with a clean wood surface exposed. This keeps resin from going through the seams.
The vac bag opens a whole bag of tricks for making boards. Most high end wind surf boards are laminated in a vacuum bag for strength. You can get the resin to glass ratio down to 1:1 and many times you will have a nearly finished surface. I haven’t laminated a board in the bag, I just use it to vacuum wood veneers onto the foam.
One thing I do occasionally is to glass the board with just a single layer of glass on the deck, then add the wood skin in the vac bag, and laminate another layer of glass over the wood. For a true Composite Sandwich, you want fiberglass on both sides of the wood.
Now after all this, let me say that if you use too soft of a foam for the core, you will eventually have failure and either soft spots, or a broken board. The oldest of my soft foam core compsand boards have many soft spots. I don’t know if they are delaminations or just the foam being smashed down. But I made those boards in 2005 or 2006 and probably got more than 5 years of use before I noticed the soft spots. I still have those boards, but I rarely use them.
Good luck!